Dampers (shock absorbers or vibration attenuators) are widely used in machines and apparatuses to reduce an impact or attenuate a vibration or a movement. Generally, the damper utilizes elasticity or viscosity of a rubber, a spring, air, or oil contained in the damper for absorbing the kinetic energy of the impact. For strengthening the power for kinetic energy absorption (damping power), usually the rubber contained is increased or the spring contained is made larger. Therefore the powerful damper is necessarily larger in size.
In vehicles, a headrest is installed on the vehicle seat to protect a sitting person from an accidental impact. The headrest is designed to move forward on a rear end collision to protect the head of the sitting person. (See for example Patent Documents 1 and 2.) Such a constitution is complicated and costly.    (Patent Document 1) Japanese Patent Application Laid-Open No. Hei-10-181403    (Patent Document 2) Japanese Patent Application Laid-Open No. Hei-10-006919